More troubleshooting:
From my Nextcloud server I ran this command and received the error seen below:
sudo -u apache ./occ onlyoffice:documentserver --check
Error connection: Error occurred in the document service: Error while downloading the document file to be converted.
I confirmed I still receive the error after adding this to my Nextcloud config.php:
'onlyoffice' => array (
'verify_peer_off' => true
)